1. Field
The present invention relates generally to medical treatment using focused radiation, and more particularly to systems for delivering such treatment.
2. Description
Conventional radiation treatment typically involves directing a radiation beam at a tumor located within a patient. The radiation beam is intended to deliver a predetermined dose of treatment radiation to the tumor according to an established treatment plan. The goal of such treatment is to kill tumor cells through ionizations caused by the radiation.
In conventional radiation treatment systems, a linear accelerator generates a divergent beam of photons having energies in excess of 1 MeV. A patient is positioned such that the beam is directed toward a treatment area of the patient. The beam may be shaped by beam-shaping devices before reaching the treatment area in an attempt to ensure that beam closely matches the shape of the treatment area and does not harm healthy tissue. Accordingly, an oncologist plans conventional treatment with megavoltage radiation by considering the divergence of the beam, the distance over which the beam travels to the treatment area, and known data representing organs and other structures internal to the patient.
Treatment systems using kilovoltage radiation may possess several advantages over the above-described treatment systems using megavoltage radiation. For example, the beam-generating hardware used in kilovoltage treatment systems may be much cheaper, simpler and more reliable than that used in megavoltage treatment systems. Environmental safety is of less concern with kilovoltage treatment systems, which typically require 3 mm of lead shielding as opposed to the 2 m of concrete shielding required for megavoltage treatment systems. Moreover, kilovoltage radiation may be more precisely targeted because it engenders less penumbra and less target overshoot than megavoltage radiation.
Despite the foregoing, megavoltage radiation has been preferred over kilovoltage radiation for use in radiation treatment. One reason for this preference is the difference in tissue-damaging mechanisms associated with each type of radiation. Megavoltage radiation damages tissue mostly through high-energy electrons created by Compton scattering of high-energy photons. These electrons penetrate inward from the site of radiation/tissue interaction, causing little damage to tissue at the surface of the interaction (e.g., skin) relative to the damage caused at a deeper target area.
In contrast, most tissue damage caused by kilovoltage radiation results from photoelectric absorption. For a given energy per unit area, damage resulting from photoelectric absorption is greatest at the surface of a radiation/tissue interaction and decreases with depth into the tissue. Consequently, a kilovotage radiation beam of uniform or decreasing energy per unit area (i.e., a divergent beam) may cause greater tissue damage at a patient's skin than at a treatment area within the patient's body.
Many techniques exist for addressing this drawback of kilovoltage radiation treatment. A kilovoltage radiation treatment system such as those described in U.S. Pat. No. 6,366,801 to Cash et al uses a radiation source which produces a divergent beam of traditional medical x-rays having energies in the 50 to 150 keV range and focuses the beam on a target area using a lens designed for this purpose. By focusing the radiation, the energy per unit area increases with proximity to the target area. As a result, tissue damage at a portion of the target area may be greater than tissue damage at a same-sized portion of the radiation/skin interaction site. Attempts to increase this relative difference in tissue damage include developing lenses that focus the radiation at greater and greater angles of convergence and/or injecting radiation-absorbing contrast agents at the target area.
Also proposed are methods in which a patient is positioned, a target area is irradiated by a radiation beam, the patient is repositioned such that a subsequent radiation beam would intercept an area of the patient's skin that was not irradiated by the previous radiation beam, and the target is irradiated again. The patient may be repositioned and the target irradiated several times. Still other methods include moving the radiation beam so as to scan the target area. None of these attempts have proved to be satisfactorily efficient and/or effective.